Thesis (MSc)--Stellenbosch University, 2005. / ENGLISH ABSTRACT: Sunburn on apple and pear fruit results in large losses of export volumes in South African orchards. Producers estimate that losses due to sunburn are often about 10- 20%. Knowledge of temperature thresholds for sunburn as well as technologies available to manage sunburn is important to increase producer earnings. The effect of evaporative cooling on fruit surface temperature and sunburn was evaluated on 'Cripps' Pink', 'Royal Gala' and 'Fuji' apples as well as 'Forelle' pears. Evaporative cooling was effective in reducing fruit surface temperature by 0.6-3.7°C. Apple fruit under evaporative cooling treatments appeared to have acclimatised and had significantly higher surface temperatures when dry (0.9-4.7°C) than non-cooled fruit. This acclimitasing effect could be due to a modification of the cuticular wax layer. Futher trials are needed to investigate the cause of this acclimatising effect. The acclimatising effect was not found on 'Forelle' pears. Sunburn was reduced significantly only on 'Royal Gala' apples. Spraying 'Braeburn', 'Granny Smith', and 'Fuji' apples with reflective kaolin-based particle film (Surround WP) significantly reduced sunburn on 'Braeburn', 'Granny Smith' and 'Fuji' apple cultivars. Surround WP applications reduced leaf stomatal conductance and photosynthetic rate. Four applications of Surround WP with a commercial blower sprayer increased total soluble solids in 'Braeburn' . Four applications with a blower and five applications with a tower sprayer resulted in increased fruit size, higher flesh firmness, and lower percentage starch breakdown in 'Granny Smith'. Surround WP treatments slightly improved fruit colour in 'Granny Smith' apples. Futher trials are needed to establish whether Surround WP can,be reliably and economically removed from apple fruit and to evaluate the storage quality of fruit cleansed in this way. The effect of fruit surface temperatures (24°C, 30°C, 33-35°C, 40-43°C and 48-49°C) for increasing periods of time (two, four, six or eight hours) on maximum quantum yield of fluorescence (Fv/Fm) was measured on exposed and shaded sides of detached 'Cripps' Pink' and 'Royal Gala' apples. Measurements were taken directly after removal from the chamber as well as 12 hours later to assess recovery. Permanent damage to Photo system II (PSII) occurred between 48-49°C irrespective of duration, with permanent damage possible at 42-43°C when exposure exceeded six hours. There were no meaningful differences in Fv/Fm between shaded and exposed sides of apple fruit after exposure to high temperature. Duration of exposure was important in lowering PSU efficiency in 'Cripps' Pink' but less so in 'Royal Gala'. 'Cripps' Pink' apples harvested later in the season appeared to be able to recover from heat stress better than apples harvested earlier in the season. However, we only studied the effects of high temperature on apple tissues. More research is needed to establish the reaction of apple tissues to high temperatures in the presence of sunlight. / AFRIKAANSE OPSOMMING: Sonbrand op appel en peervrugte kom jaarliks voor in Suid-Afrikaanse boorde en lei tot groot afnames in vrugte wat geskik is vir die uitvoermark. Produsente beraam dat verliese a.g.v. sonbrand jaarliks 10-20% beloop. Kennis van drempelwaarde temperature vir die ontwikkeling van sonbrand sowel as die tegnologieë wat beskikbaar is om sonbrand te bekamp, is van groot belang om uitpakke te verbeter. Die effek van oorhoofse besproeïng op vrugtemperature en sonbrand van 'Cripps' Pink', 'Royal Gala' en 'Fuji' appels sowel as 'Forelle' pere is ondersoek. Oorhoofse besproeïng het vrugtemperature betekenisvol verlaag in al die proewe. Dit wil voorkom of appels onder oorhoofse besproeïng geakklimatiseer het en dié appels het hoër skil temperature geopenbaar wanneer hulle droog was as die kontrole vrugte. Moontlik is die effek te wyte aan 'n verandering in die kutikulêre waslaag. Verdere proewe om hierdie verskynsel te probeer verklaar is nodig. Die effek is nie waargeneem op die 'Forelle' pere nie. Sonbrand is betekenisvol verminder op 'Royal Gala' appels. Toediening van weerkaatsende kaolin-gebaseerde partikels (Surround'" WP) het sonbrand betenisvol verlaag op 'Braeburn' , 'Granny Smith', en 'Fuji' appels. Surround® WP toedienings het blaarhuidmondjie-geleiding en die tempo van fotosintese verlaag. Vier toedienings van Surround® WP met 'n kommersiële waaierblaser het totale oplosbare vastestowwe (TOVS) beduidend verhoog in 'Braeburn' appels. Vier toedienings met 'n waaierblaser en vyf toedienings met 'n toring spuitkar het gelei tot 'n verhoging in vrugmassa en vrugferrnheid, en 'n verlaging in persentasie styselafbraak in 'Granny Smith' appels. Surround® WP toedienings het gelei tot effens beter kleur op 'Granny Smith' appels. Verdere proewe is nodig om te bepaal of Surround® WP op 'n betroubare en ekonomiese wyse van vrugte verwyder kan word in die pakstoor, en wat die hou-vermoë is van appels wat só behandel is. Die effek van vrugoppervlaktemperature (24°C, 30°C, 33-35°C, 40-43°C en 48-49°C) vir toenemende tydsdure (twee, vier, ses en agt ure) op maksimum kwantum-opbrengs van fluoressensie is gemeet op sonkante en skadukante van geplukte 'Cripps' Pink' en 'Royal Gala' appels. Metings is geneem direk na verwydering uit die oonde sowel as 12 ure later om moontlike herstel waar te neem. Permanente skade aan Fotosisteem II (PSII) het plaasgevind tussen 48-49°C ongeag tydsduur, met permanente skade ook moontlik by 42-43°C wanneer blootstelling aan genoemde temperature ses ure oorskry het. Daar was geen betekenisvolle verskille in Fv/Fm tussen skadu- en sonkante van appels na blootstelling aan hoë temperature nie. Die tydsduur van blootstelling was belangrik om PSII effektiwiteit te verlaag in 'Cripps' Pink', maar minder so in 'Royal Gala'. Dit wil voorkom of 'Cripps' Pink' appels later in die seisoen beter daartoe in staat is om te herstel van hitte stres as vroeër in die seisoen. Ons het egter slegs die effek van hoë temperature op appel weefsels ondersoek. Verdere proewe is nodig om die reaksie van appelweefsels op hoë temperature in die teenwoordigheid van sonlig te ondersoek.

Thesis (MScAgric)--University of Stellenbosch, 2002. / ENGLISH ABSTRACT: Daily daylight fertigation regimes (involvinq drip fertigation, on a daily basis, during the daylight hours) are becoming widely used in the South African citrus industry in the endeavor to enhance tree productivity. Such regimes could provide sensitive nutrient and moisture management, reducing plant stress in general as well as the response time to root-directed cultural activities. There is a need to evaluate the efficacy of daily daylight fertigation systems relative to conventional irrigation systems. Standard horticultural evaluation of orchard management practices is very time consuming. We opted for physiological studies comparing plant stress levels, in an attempt to quantify plant performance under each system. Citrus trees under daily daylight fertigation and conventional micro-jet and drip-irrigated regimes were monitored to establish plant stress levels as indicated by sap flow, xylem water potential, stomatal conductance and chlorophyll a fluorescence. Plants under a daily daylight fertigation regime are believed to have good soil water conditions in their rooting volume, and therefore experience negligible baseline levels of stress. The trees do, however, experience midday depression in stomatal conductance, to a lesser degree, but not unlike trees under conventional regimes. It appears as if a larger rooting volume of microjet irrigation regimes enhances recovery from the midday depression. It is recommended that producers optimise productivity during the moming hours, by early irrigation, so that plants can function optimally, whilst environmental conditions are most favourable for high physiological activity. We also assessed the effect of withholding water from trees adapted to a daily daylight fertigation regime to evaluate the risk involved with short-term water deficits in trees adapted to this regime, as well as the usefulness of physiological techniques for identifying water stress. Stomatal conductance and xylem water potential indicated water stress sooner than the other physiological parameters. Citrus trees seem to be relatively insensitive to water deficit stress as measured by sap flow and chlorophyll a fluorescence. Sap flow is buffered by tree capacitance, and although mediated via stomatal conductance, atmospheric conditions and not the soil water content primarily determine it. As daily fertigation is applied to trees under DOF regimes, they exhibit more optimal levels of xylem water potential and stomatal conductance, compared to trees from which water is withheld. Although alleviating it to a degree, daily irrigation did not mitigate the midday depression in these values. Seen over a season, even small enhancements of stomatal conductance (and with it photosynthesis and possibly, growth) and xylem water potential, could incrementally produce higher yields. / AFRIKAANSE OPSOMMING: In die strewe na verhoogde boomproduktiwiteit, word daaglikse sproeibemesting (deur 'n drupbesproeiingsstelsel toegedien tydens die dagligure) al meer algemeen in die Suid Afrikaanse sitrusbedryf gebruik. Hierdie praktyk verminder algemene plantstres deur baie spesifieke voedingsen vogbeheer, en verkort ook die plant se reaksietyd op wortelgerigte bewerkingsaktiwiteite. Dit is nodig om die relatiewe voordeel van daaglikse sproeibemesting teenoor konvensionele besproeiingssisteme te evalueer. Huidige tuinboukundige evaluering van boord-bestuurspraktyke is baie tydrowend. In 'n poging om plantreaksie onder verskillende praktyke te beskryf, het ons besluit om die plantstresvlakke met fisiologiese metodes te vergelyk. Sitrusbome onder daaglikse sproeibemesting, en konvensionele mikro- en drupbesproeiing, is onderskeidelik gemonitor om die plant se stresvlakke vas te stel, soos aangedui deur sapvloei, xileem-waterpotensiaal, stomatale geleiding en chlorofil a f1uoresensie. Die plante onder daaglikse sproeibemesting ondervind lae viakkke van waterstremming, waarskynlik weens hoë grondvogtigheid in die wortelsone. Die bome ondervind wel, soos dié onder konvensionele besproeiing, middagdepressie in stomatale geleiding, hoewel tot 'n mindere mate. Dit blyk asof die groter wortelvolume van mikrospuit besproeide bome die herstel na middagdepressie bespoedig. Produsente word aangeraai om die oggendure optimaal te gebruik deur vroeg te besproei sodat plantproduktiwiteit hoog is terwyl die omgewingsfaktore op hul gunstigste is en wanneer die hoogste fisiologiese aktiwiteit voorkom. Ons het ook die effek van wateronthouding gemeet op die bome wat aangepas is vir daaglikse sproeibemesting. Sodoende is die risiko verbonde aan 'n korttermyn watertekort op hierdie bome ge-evalueer, asook die bruikbaarheid van fisiologiese tegnieke om waterstremming in sitrus te identifiseer. Stomatale geleiding en xileem-waterpotensiaal het waterstremming vroëer aangedui as die ander fisiologiese parameters. Sitrusbome blyk redelik onsensitief te wees teenoor droogtestremming soos gemeet deur sapvloei en chlorofil a fluoresensie. Sapvloei word gebuffer deur boom-kapasitansie, en alhoewel sapvloei gereguleer word deur stomatale geleiding, is dit die atmosferiese toestande (hoofsaaklik dampdruk verskil) wat dit primêr beïnvloed, en nie die grond-water inhoud nie. Omdat bome daagliks sproeibemes word, het hulle meer optimale vlakke van xileemwaterpotensiaal en stomatale geleiding in vergelyking met bome waarvan water weerhou is. Alhoewel daaglikse sproeibemesting die middagdepressie verlaag het, is dit nie daardeur opgelos nie. Oor die typerk van 'n seisoen kan selfs minimale verhogings in stomatale geleiding (en daarmeesaam fotosintese en moontlik groei) en xileem-waterpotensiaal, hoër opbrengste tot gevolg hê.

Thesis (MScAgric)--University of Stellenbosch, 2007. / ENGLISH ABSTRACT: The macro-element uptake and distribution by higher density central leader ‘Donnarine’ nectarine trees was studied through the sequential excavation of trees. A large portion, 41.5%, of the nitrogen manifested in the new growth from dormancy up to pit-hardening, originated from nitrogen reserves in the permanent structure. The permanent structure was also an important source of phosphorous reserves. Of the phosphorous in the fruit, leaves and new shoots at pit-hardening, 35.0% came from reserves in the permanent structure. Potassium did not act as an important reserve in the nectarine trees and was taken up throughout the season. From pit hardening to harvest the fruit represented the most important sink for potassium. Calcium and magnesium, like potassium, did not play significant roles as reserves in the nectarine tree and must be available for uptake from the beginning of the season for new growth and development as well as fruit quality. The micro-element uptake and distribution was also studied through the sequential excavation of the same ‘Donnarine’ nectarine trees. Little scientific data is available on this topic. Manganese and iron was found to act as important reserves in the tree with 46.2% of manganese and 59.5% of the iron found in the new growth at pithardening coming from reserves translocated from the permanent structure. Zinc and boron reserves also play a role in nectarine trees, but to a lesser extent than manganese and iron. The seasonal mineral nutrient demand of the same ‘Donnarine’ nectarine trees was determined through the sequential excavation of trees and losses and fixation was calculated. Guidelines regarding nutritional requirements per ton of fruit produced per hectare by higher density nectarine orchards are respectively 3.82kg nitrogen, 0.35kg phosphorous, 4.43kg potassium, 1.53kg calcium, 0.52kg magnesium, 32.45g sodium, 9.44g manganese, 37.46g iron, 3.24g copper, 13.95g zinc and 10.52g boron. Sodium is not commonly considered to be essential to higher plants, but was included in the trial. Nutrient solutions with four different EC (electrical conductivity) levels were applied to ‘Donnarine’ nectarine trees under pulsating drip fertigation for three periods of different lengths, before harvest. Raising the nutrient solution EC to positively affect fruit quality is a technique widely utilised in the vegetable industry. This technique did, however, not have similar positive effects on nectarine fruit grown under a pulsating drip fertigation system. Good production practices such as accurate nutrition and irrigation as well as the correct horticultural inputs should be the primary focus of producers who wish to alter or improve the fruit quality of their crop. / AFRIKAANSE OPSOMMING: Voedingbestuur strategieë in nektarien boorde Die makro-element opname en verspreiding deur hoër digtheid sentrale leier ‘Donnarine’ nektarien bome is bestudeer d.m.v. opeenvolgende opgrawings van volledige bome en die ontleding van monsters. ‘n Groot hoeveelheid, 41.5%, van die stikstof wat tydens pitverharding in die nuwe groei teenwoordig was, is d.m.v. translokasie vanuit die permanente struktuur van die boom afkomstig. Die permanente struktuur was ook ‘n belangrike bron van fosfaat reserwes. Teen pitverharding was 35.0% van die fosfaat in die nuwe groei afkomstig vanuit die permanente struktuur. Bevindings het getoon dat kalium nie as ‘n reserwe in die nektarien bome opgetree het nie en dié element is deur die groeiseisoen opgeneem. Vanaf pitverharding tot en met oestyd was die vrugte die sterkste setel van aanvraag vir kalium. Kalsium en magnesium het, soos in die geval van kalium, nie ‘n belangrike rol as reserwe vertolk nie. Beskikbaarheid van hierdie elemente vir opname vanaf die begin van die groeiseisoen is dus baie belangrik vir nuwe groei en ontwikkeling asook vrugkwaliteit. Die mikro-element opname en distribusie van dieselfde ‘Donnarine’ nektarien bome is ook bestudeer d.m.v. opeenvolgende opgrawings en analise van volledige bome. Min wetenskaplike literatuur oor hierdie onderwerp is beskikbaar. Bevindings het getoon dat mangaan asook yster baie belangrike reserwes in die nektarien boom is. Tydens pitverharding was 46.2% van die mangaan en 59.5% van die yster wat in die nuwe groei teenwoordig was, afkomstig vanaf reserwes uit die permanente struktuur van die boom. Verder het sink en boor ook as reserwes opgetree, maar tot ‘n mindere mate as mangaan en yster. Die seisoenale behoeftes aan minerale voeding van dieselfde ‘Donnarine’ nektarien bome is bepaal d.m.v. opeenvolgende opgrawings en analise van volledige bome asook die bepaling van verwyderingsverliese en vaslegging. Voedingsriglyne is vasgestel i.t.v. die hoeveelheid voedingstof wat per hektaar benodig word om een ton nektariens te produseer. Die riglyne is as volg: 3.82kg stikstof, 0.35kg fosfaat, 4.43kg kalium, 1.53kg kalsium, 0.52kg magnesium, 32.45g natrium, 9.44g mangaan, 37.46g yster, 3.24g koper, 13.95g sink en 10.52g boor. Natrium word nie in die algemeen as ‘n essensiële plantvoedingselement beskou nie, maar is by die berekeninge ingesluit. Voedingsoplossing met vier verskillende vlakke van EG (elektriese geleiding) is vir drie periodes van verskillende lengtes aan ‘Donnarine’ nekarien bome toegedien. Die verhoging van die EG van voedingsoplossings ten einde kwaliteit te verbeter is ‘n tegniek wat met groot suskses in die groentbedryf toegepas word. Hierdie tegniek het egter nie soortgelyke positiewe effekte op die nektarien vrugkwaliteit gehad nie. Produsente wat hul vrugkwaliteit wil verbeter behoort primêr te fokus op goeie produksiepraktyke soos akkurate voeding en besproeiing asook die korrekte tuinboukundige insette.

Thesis (MscAgric (Horticulture))--University of Stellenbosch, 2007. / Production of quality fruit is the main aim in horticultural crops. Numerous research reports stress the important role of calcium (Ca) in maintaining firmness and preventing the development of physiological disorders in fruit. This study focused on the effect of water and nutrient management strategies, rootstocks and foliar Ca applications on fruit Ca content. Final Ca content/concentration in apple fruit at harvest did not differ significantly between treatments water with micro jets (hand nutrition), water and nutrients with fertigation, or water and nutrients with pulsating drip when applied to ‘Brookfield Gala’ trees in third leaf, on two rootstocks (M793 and M7). In the second trial, three Ca levels were applied to ‘Brookfield Gala’ trees through a pulsating drip system during three phenological periods to evaluate the effect on Ca uptake of the fruit. During the second season, application of high Ca levels for the period full bloom to harvest gave a higher fruit Ca concentration than with applications of standard or low Ca.

Thesis (MSc)--Stellenbosch University, 1955. / ENGLISH ABSTRACT: no abstract available / AFRIKAANSE OPSOMMING: geen opsomming

Thesis (MSc (Horticulture))--University of Stellenbosch, 2006. / A growing number of fruit producers in warm areas are adopting the use of overtree evaporative cooling (EC) as a technique to reduce sunburn and enhance colour development of red or blushed fruit. Because fruit do not have efficient mechanisms of utilising and/or dissipating solar radiation, fruit surface temperature may rise 10 – 15oC higher than the ambient air temperature, making them very susceptible to sunburn. Sunburn negatively affects the appearance of the fruit, and they cannot be sold for fresh market consumption, which receives the highest prices. Evaporative cooling uses a sprinkler system to cool the trees from above. Energy needed to evaporate the water is extracted from the fruit skin, cooling the fruit down. The air around the trees is cooled, and a more favorable microclimate is created in the orchard. Producers have also found that the use of EC just prior to sundown and sometimes around sunrise has improved colour development on red apples (especially early varieties) before harvest. In this study, two apple (‘Cripps’ Pink’ and ‘Royal Gala’) and two pear (‘Rosemarie’ and ‘Forelle’) cultivars under EC were compared with control fruit in terms of maturity, colour, sunburn and concentrations of polyphenolics in the skin. Two EC treatments were given; early application starting from the second week in December, and late application starting two to four weeks before harvest. Photosynthetic responses were measured, as well as fruit and leaf temperatures. Underlying physiological responses of trees and fruit to EC were investigated, particularly the phenomenon of acclimation and the potential for colour development and heat stress. Fruit surface temperature of fruit under EC was found to be significantly lower than control fruit. In both apple cultivars a significant increase in fruit skin anthocyanin concentration and a decrease in phenolic content was found as the season progressed. In both pear cultivars there was a significant decrease in both anthocyanin and phenolic. No significant differences were found in anthocyanin content between treatments in either the apple or pear cultivars. In both apple cultivars a higher phenolic content was found in the peel of the EC treatments. A decrease of up to four percent in leaf and fruit surface temperature was found under EC. No significant difference in trunk circumference was found in any of the cultivars. The late EC treatment in ‘Cripps’ Pink’ had a significantly faster rate of budbreak than the control and early EC treatments. Significantly higher transpiration was observed in leaves under EC. ‘Royal Gala’ fruit under EC had less sunburn than control fruit. Unfortunately the system broke down on a hot day, causing more sunburn on ‘Cripps’ Pink’ fruit under EC. Heat tolerance of apple fruit grown under EC was evaluated in ‘Cripps’ Pink’ and ‘Royal Gala’ by determining the maximum quantum yield of chlorophyll fluorescence (Fv/Fm). Measurements were also made 12 hours after the heat treatments to determine recovery. ‘Cripps’ Pink’ fruit from both EC treatments, but particularly the early EC treatment, were less resistant to heat stress than control (non-EC) fruit at the “threshold” air temperature of 45°C. Apples were able to recover from heat treatments in the range of 32-38oC fruit surface temperature, and generally also recovered fully after 43-45°C fruit surface temperature when exposure did not exceed four hours. This knowledge could be helpful in the management of sunburn, for example when determining the threshold temperature for the activation of evaporative cooling treatments. Knowledge about the various effects evaporative cooling and the subsequent lowering of ambient temperatures has on fruit trees and fruit could contribute greatly to producers’ ability to grow high quality fruit. EC can be used successfully for controlling sunburn and increasing fruit colour, but the system needs to be controlled very carefully and care should be taken that it does not fail on a hot day, as it did during this study.